In certain implementations, especially in circuits for switching the reading heads of video tape recorders which require amplifiers with differential outputs, it is compulsory for obtaining good operating conditions of the downstream circuit (switch), which receives the output signals from the amplifier, that those output signals do not present any d.c. level. On the other hand, it is also compulsory that there is not a large difference between the d.c. levels or at least that this d.c. level difference be as low as possible. Those offset voltages may have various origins, and are generally not clearly determined. The usual way to solve this problem then consists in locating on the amplifier signal input a serial capacitor, this capacitor is then located between the upstream circuit and the amplifier. The purpose of the capacitor is to cancel the offset. However, not all the causes of the offset are cancelled. A d.c. level appears more particularly on each of the outputs, this level varies in a non-determined way and is amplified at the outputs.
As a matter of fact, the differential amplifier conventionally comprises two transistors biased by the d.c. component of the input signal. When a serial capacitor is used at the signal input, the d.c. component of the signal is suppressed. It is then necessary to bias the base of the transistors. To achieve this purpose, a reference source is used as well as a resistor connected between this source and the base of each of the amplifying transistors. always Because it is impossible to get resistors strictly identical in value, a offset occurs at the amplifier outputs. This offset is amplified at the output by the gain provided by the circuit.
On the other hand, in order to bias the transistors, it is necessary to use resistors with a high value which, of course, add some noise. To avoid adding more noise when using resistors with high value, resistors with lower value are used as well as capacitors with higher value. One is then faced with the problem caused by the integration of such capacitors. It is then necessary to choose between noise and integration. One generally uses a capacitor with a high value (but not integrated) connected with the housing of the integrated circuit by means of two connecting pins of the housing.
The problem to solve involves providing a differential amplifier which does not present any d.c. level variation between its outputs, which can be integrated, which does not add any noise and which further avoids the use of two housing connection pins.